| This paper mainly studies the technological basis of gas blow forming for AZ31/7475 bimetal composite stepped tube.The process is to make the two tubes synchronously bulge in the heat forming temperature range in which the two alloys overlap,and the clamping force is generated due to their different shrinkage amount after the forming,so that the two tubes are tightly combined.Through the combination of numerical simulation and experiment,the gas blow forming process of the stepped tube and the quality of the formed parts were systematically analyzed.The gas blow forming process of the stepped tube is divided into three stages: a free bulging stage,a mold clamping stage,and a shaped filling stage.The finite element simulation software ABAQUS was used to simulate the three stages of the gas blow forming process of the bimetallic composite stepped tube using the quadrilateral shell element S4 R.The initial air pressure in the free bulging stage,the axial feeding in the mold clamping stage and the final air pressure in the shaping filling stage were analyzed.The appropriate forming parameters were determined as follows: the initial air pressure is 1.6 MPa,the axial feeding is 10 mm and the final pressure is 4.5 MPa.The variation law of stress and strain during forming process was analyzed.The wall thickness distribution law of the stepped tube was studied.The influence of axial feeding on the minimum wall thickness and wall thickness unevenness was analyzed.The forming danger points of the magnesium tube and the aluminum tube are respectively located at the intersecting point of the first step straight segment and the rounded transition and the midpoint of the tube.Taking the numerical results as a guide,a set of gas blow forming mold for stepped tube was designed and the gas blow forming experiments of AZ31/7475 bimetal composite stepped tube were carried out at 440 ?,and some magnesium/aluminum bimetal composite stepped tube meeting the requirements were formed successfully.The effects of initial air pressure,axial feeding and final air pressure on the forming process were obtained.The wall thickness distribution of the formed parts and the influence of axial feed on the wall thickness distribution were investigated.It was found that the axial feed was appropriately increased,the wall thickness distribution of the stepped tube can be improved significantly,and the experimental results also verify the accuracy of the simulation results.The causes of four typical forming defects namely cracking,wrinkling,under fill and flashing were analyzed,and preventive measures were proposed.The interface between the magnesium/aluminum of the bimetal composite stepped tube is clear,there is no diffusion phenomenon,and there is no metallurgical bond.The residual contact stress between the magnesium tube and the aluminum tube is composed of the cold shrinkage residual contact stress and the rebound residual contact stress,and the total contact stress for the first step,the second step and the base tube of the stepped tube were calculated as 3.45 MPa,3.60 MPa,and 3.51 MPa,respectively.The influence of height-diameter ratio,thickness-diameter ratio and number of tube blanks on feed compression instability in tube bulging deformation zone was simulated by ABAQUS finite element software.With the increase of the thickness-diameter ratio,the critical initial pressure required to expand the single drum shape after the mold clamping is linearly increased;the critical initial pressure of the double-layer composite tube is much larger than that of the single-layer tube,but as the height to diameter ratio changes,the initial pressure value does not change significantly.;the thickness-diameter ratio increases,and the critical initial gas pressure required to bulge the single drum shape after clamping is increased. |
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